Fix worked for the HL-2140 series. It might work for other printers, not sure on that.
1. Turn off printer
2. Open up the front
3. Take out the Drum Unit
4. Turn on printer
5. Put the Drum back on and close the front
6. Error light should be off by now and you will be able to print again
Source:
- Pictures: http://www.scribd.com/doc/48551012/Brother-HL-2140-drum-reset
- Fix: http://www.fixyourownprinter.com/forums/laser/63546